1. Field of the Invention
The invention relates to a process for the treatment of tobacco, in particular Burley tobacco, by the application of casing and subsequent thermal treatment.
2. Discussion of Prior Art
The application of so-called casing is a customary procedure in the preparation of leaf tobacco before cutting. The aim of a casing treatment is the improvement of the processability as well as the taste properties of the tobacco material. Normal constituents of casings are humectants such as e.g. glycols or glycerol, sugar and solid natural substances such as for example cocoa or licorice. High viscosity and the solids content of casings make a desired, even penetration of the tobacco product by the casing components more difficult. In order to improve the penetration of the casing into the cell structure of the tobacco, the application of the heated casing is carried out together with a moistening by water and steam in units called casing drums. The conventional technique of casing application is described in Voges, “Tobacco Encyclopedia”, Mainzer Verlagsanstalt und Druckerei Willi und Rothe GmbH & Co K G, Mainz, 1984; p. 65 (Keyword “Casing”), p. 411 (Section on “Tobacco Flavours and Casings”), pp. 416 and 417 (Section on “The Production of Cut Tobacco”, passages headed “Special Treatment for Burley” and “Casing”).
As a rule, Burley tobaccos combine relatively high levels of nitrogen compounds and low sugar contents. For this reason, an acceptable smoke taste can often be achieved only by using sugar-containing casings in combination with a subsequent thermal treatment. Along with a removal of volatile nitrogen compounds by the thermal treatment, reaction products such as e.g. pyrazines can form from sugar and nitrogen components which contribute to the improvement of the sensory quality. As a rule, a so-called belt dryer with several drying and cooling zones is used for the thermal treatment of casing-treated Burley tobacco, the tobacco being dried from approx. 30% initial moisture content to approx. 5% moisture content. For further processing, in particular for cutting, the tobacco has to be moistened again to approx. 16% to 22%.
This conventional procedure has several disadvantages. Firstly, in the case of application through a casing drum, the penetration of the casing into the leaf material is not optimal, due to the low action intensity of the steam. Furthermore, much energy has to be expended for the strong drying. Belt dryers also occupy a lot of space and, because of their design, lead to an inhomogenous moisture distribution. A further disadvantage is the high level of fragility of the tobacco material at moisture levels under 10%, which leads to losses through formation of tobacco fines and dust.
For this reason several processes have been described which are said to circumvent one or more disadvantages of the conventional treatment method.
U.S. Pat. No. 5,755,238 describes a method for quick drying, cooling and remoistening using a drying unit divided into several treatment zones and remoistening in a separate steam tunnel. The moisture content of the tobacco material is approx. 30% before drying, approx. 5% before the steam tunnel and approx. 15% after the remoistening. The hot air temperature of the dryer is approx. 105° C. to 115° C. and the total passage time approx. 60 seconds. By using several fluidized-bed drier zones an improved moisture homogeneity is said to be achieved.
U.S. Pat. No. 4,004,594 discloses a method for conditioning tobacco, in particular Burley tobacco, which provides for an impregnation of the tobacco particles with casing, a thermal treatment to expel the nitrogen or the nitrogen compounds and the adjustment of the desired moisture content. The treatment plant consists of a dosing unit, a casing drum, a preconditioning unit for the treatment with steam and units for the heating, cooling and remoistening of the tobacco. The tobacco moisture contents are 14% to 20%, preferably 18%, after the dosing unit, 30% to 42%, preferably 32%, after the casing drum, approx. 35% after preconditioning, 4% to 7% after heating and approx. 18% to 22% after remoistening. In the preconditioning unit a treatment with saturated steam of 2.5 bar to 3.5 bar takes place, resulting in a tobacco temperature of approx. 70° C., in order to achieve an improved penetration of the casing into the tobacco leaf.
U.S. Pat. No. 3,402,479 describes a tunnel-like apparatus for transporting and treating nitrogen-rich tobacco with zones for predrying, for heating the tobacco without loss of moisture by using a correspondingly conditioned medium and for cooling the treatment product. The moisture content of the tobacco material upon entry is 40% to 50%, the temperature of the medium after the predrying approx. 100° C. and the moisture on leaving approx. 16% to 18%. The use of this process, costly in terms of apparatus, in combination with the extremely high moisture contents upon entry is intended to avoid an overdrying of the tobacco material and the associated increase in fragility.